1. Field of the Invention
The present invention generally relates to a sheet conveyance unit used in an image forming apparatus, such as a copier, a printer, a facsimile machine, or a multifunction machine including at least two of these functions, and an image forming apparatus including the sheet conveyance unit.
2. Discussion of the Background Art
In image forming apparatuses such as laser printers, generally the following processes are performed.
A sheet feeder picks up sheets of recording media stored in a sheet cassette one by one and feed the sheet to a transfer position, and a toner image is formed on an image bearer such as a drum-shaped or belt-shaped photoreceptor. At the transfer position, the toner image is transferred from the image bearer to the sheet with the position of the toner image on the image bearer aligned with an image position on the sheet, after which the toner image is fixed on the sheet and the sheet is output.
Such image forming apparatuses further includes a registration unit, such as those described below, to time sheet conveyance to coincide with arrival of the toner image at the transfer position so that the toner image can be transferred at an intended position on the sheet.
For example, JP-S64-0555-U proposes a registration unit that includes a pair of registration rollers to transport the sheet timed to coincide with the arrival of the toner image, and a movable gate positioned upstream from the registration rollers in the sheet conveyance direction. The gate can move to block the sheet conveyance path and be away from the sheet conveyance path.
In this approach, the sheet is conveyed a predetermined distance with the leading edge of the sheet stopped by the gate, causing the sheet to curve into a partial loop.
When the gate is away from the sheet conveyance path, the curved sheet is stretched, and then the leading edge of the sheet can enter the nip between the registration rollers due to resilience of the sheet to return to its original shape.
The position where the leading edge of the sheet is clamped by the registration rollers can be the same or similar among multiple sheets due to the resilience of the sheet. Therefore, the timing at which the sheet is transported from the sheet cassette can be constantly in alignment with the arrival of the toner image at the transfer position among the multiple sheets transported successionally.
Although the gate is used in the above-described approach, alternatively, the leading edge of the sheet may be forced to get stuck in the nip between the registration rollers so that the sheet is curved without using the gate.
The above-described approach, however, has several drawbacks. For example, because the gate dedicated to sheet registration is provided separately from the registration rollers, the mechanism is more complicated and the cost is increased.
Additionally, JP-H05-338865-A proposes providing the registration roller with a movable contact member that stops the leading edge of the sheet. The movable contact member is a rotary gate provided via a torque limiter at a rotary shaft of the registration roller. In this configuration, as the registration roller rotates in a normal direction (i.e., sheet feeding direction) and the reverse direction, the rotary gate moves between a position to stop the leading edge of the sheet (i.e., a registration position) and a position to forward the sheet in conjunction with the registration roller (i.e., a non-contact position.
More specifically, the rotary gate moves to the registration position when the registration roller rotates in the direction reverse to the sheet feeding direction to forward the sheet and to the non-contact position from the registration position when the registration roller rotates in the sheet feeding direction. When the rotary gate contacts a sheet guide, the torque limiter does not transmit rotation of the registration roller to the rotary gate, thereby stopping the rotary gate.